(3H)-2, 3, 7, 8-TCDD has been used extensively to characterize the molecular properties of the Ah receptor-ligand complex. However, recent studies in our laboratory have indicated that this radioligand is not ideally suited for binding studies. New developments in my laboratory have resulted in methods for synthesizing several radiolabeled polychlorinated dibenzofurans and dibenzo-p-dioxins of high specific activity (35-55 Ci/mmol). The proposed studies will focus on measuring some of the fundamental properties of the new radioligand-receptor protein (rat liver) complexes and these include the following: a) saturation binding studies will be determined and the affinity constants (KD) and Rm (receptor concentrations) will be calculated for all the radioligands using a common rat hepatic Ah receptor preparation; b) sucrose density gradient centrifugation studies will be carried out using the selected radioligands (Table D1) and rat hepatic cytosol (sedimentation constants, Stokes radius, relative molecular mass (Mr) frictional and axial ratios will be calculated); c) the competitive EC50 or IC50 of all the (unlabeled) compounds listed in Table D1 will be determined using each of the radiolabeled ligands; d) the effects of high salt concentrations on the disaggregation of 9S receptor complex will be determined for all the radioligand (Table D1)-receptor complexes; e) the interactions of the receptor protein-radioligand complexes with DNA utilizing DNA-cellulose columns will also be determined; and the effects of radioligand structure on the nuclear uptake and persistence of the nuclear-radioligand- receptor complexes in mice and mouse hepatoma cells will also be investigated. In addition, a kinetic analysis of the interaction of the new radioligands with the Ah receptor will be determined and simulated models will be generated and compared with data already obtained for (3H)-2, 3, 7, 8-TCDD (Appendix, Section C). These studies will not only characterize the molecular interactions of a series of new radiolabeled PCDD and PCDF radioligands but also determine if specific alterations in radioligand structure which are known to affect their toxicity correlate with changes in any of the molecular properties of the radioligand-receptor complexes.